1. Field of the Invention
The present invention relates to a process for preparing a thermosettable polyurethane product which can be effectively obtained.
2. Description of the Prior Art
Thermoplastic polyurethane elastomers have relatively high hardness over the other elastomers as well as excellent elasticity, tensile strength and oil resistance. In view of such characteristics, thermoplastic polyurethane elastomers have been used in various fields. Thermoplastic polyurethane elastomers, however, have substantially linear structures whereby they are easily soluble in polar solvents. Moreover, thermoplastic polyurethane elastomers have large regenerative property in continuous repeating frictions whereby the friction contacting parts are disadvantageously heat-melted. It has been known to desire a crosslinking structure in the molecule of the polyurethane elastomer in order to overcome these disadvantages.
The following procedures have been proposed as the processes for forming crosslinked polyurethane elastomers having high quality.
(1) As it is found in a casting process, a crosslinked polyurethane elastomer is produced by reacting, a linear polyester or polyether having hydroxyl groups with an excess of an organic diisocyanate to be required for the reaction with hydroxyl groups.
(2) A crosslinked polyurethane elastomer is produced by mixing a solution of a crosslinking agent having 3 or 4 functional terminal NCO groups in a solvent to a solution of a substantially linear polyurethane in a solvent and coating the mixture on a parting paper or impregnating the mixture in a fabric and evaporating the solvent in a dryer.
In the process (1), high technique under long experience and high skill is required and accordingly, the process (1) is not suitable for a continuous fabrication for preparing electric wires and tubes by an extrusion. In the process (2), a solvent is used and accordingly, the process (2) is not suitable for preparing a small size fabricated product except it is prepared by a coating process.